The invention relates to cosine transform computing devices, and more particularly to such devices which are used for image coding and decoding in order to reduce the amount of information representing these images.
Coding the values representative of the luminance and of the color of the points of an image is known using a transformation called a cosine transformation which, this transformation causes a matrix of N.times.N values, called transforms, to correspond to a matrix of N.times.N values, representing points of a portion of an image to be coded. This image is cut up into square portions each formed by a block of N.times.N points. Weighting the transformed values allows the amount of information representative of an image to be reduced. Decoding consists in applying a reverse weighting, then a reverse transformation, which causes a matrix of N.times.N reverse transformed values to correspond to a matrix of N.times.N forward transformed values. Like the coding, decoding is achieved by blocks of N.times.N image points. If the values representative of the points of a block are f(i, j) for i=O to N-1 and j=O to N-1, the values of the forward transforms are given by the following formulae: ##EQU1##
The reverse transformed values are supplied by the reverse cosine transformation by applying the following formulae: ##EQU2##
This cosine transformation is a bidimensional transformation which may be broken down into two monodimensional cosine transformations and the bidimensional transforms may be computed using two monodimensional transform computing devices connected in cascade. The monodimensional cosine transformation is obtained in accordance with the following formula: ##EQU3##
The reverse monodimensional cosine transformation is obtained by the following formulae: ##EQU4##
Coding of an image by cosine transformation followed by weighting allows the amount of information to be transmitted to be greatly reduced, but has the drawback of requiring very numerous calculations. Such a large amount of calculation is costly in equipment and in computing time. It makes it very difficult to apply cosine transformation to the coding and decoding of a succession of video images at the usual frequency, fifty frames per second in the European standards.
Integrated circuits exist on the market for performing addition or subtraction operations, or a multiplication and accumulation operations. Their performances in speed and accuracy are, however, limited. A known method for providing a large amount of calculations at high speed consists in multiplying the number of computing devices and in working them in parallel. Such transform computing devices thus formed would then be too costly to be usable in wide commercial distribution applications.